This invention relates to systems for conveying items so as to assure an adequate backlog at the output thereof.
Conveying systems of this type find particular utility in the field of packaging. In many instances, it is necessary to provide a backlog of items to a packaging apparatus to assure the proper operation of that apparatus. In the baking industry, pies and cakes and the like are conveyed from an oven to a baked goods wrapping machine which must be supplied with baked goods at a proper and continuous rate to maximize efficiency in wrapping as well as preventing jamming due to the absence of baked goods to be wrapped. For example, an FMC model 1301 wrapping machine wraps baked goods admirably so long as the baked goods are supplied to the input of the wrapping machine at a constant rate. However, in the absence of baked goods, the machine will jam resulting in a considerable down-time of the baked goods line.
In order to assure a proper supply to wrapping machines of this type, the wrapping machines have been supplied manually. This is an extremely costly way to assure an adequate supply to the wrapping. Moreover, it is very tedious work for the person or persons who are required to stand for hours on end, satisfying the demand of the wrapping machine.
The packaging of baked goods is but one example of situations wherein a constant supply must be provided at the output of the conveyor system. Filling lines for bottles and/or cans involve similar requirements. Manufacturing processes such as that disclosed in U.S. Pat. No. 2,916,792 -- Crook et al also have similar requirements.
The Crook et al patent discloses a method and apparatus for extruding soap and conveying that soap so as to provide an adequate backlog for a press. To achieve this backlog, the soap extruder is driven at a variable speed in response to the backlog accumulated from a constant highspeed conveyor coupled to the output of the extruder. The backlog is created by providing a stop at the end of the accumulator conveyor which actually halts the advancement of the bars of soap on the accumulator conveyor. Electrodes positioned above and below the accumulator conveyor senses the change dielectric constants provided by the accumulating bars of soap. This change in capacitance which forms part of a capacitance bridge is then utilized to control a servo motor which in turn controls the motor driving the soap extruder.
The Crook et al system, while perhaps adequately satisfying the demand of the soap press, is in general too slow to satisfy the needs of a high-speed packaging line. The stop at the end of the accumulator conveyor which interrupts the advancement of the bars of soap on the accumulator conveyor so as to create a backlog, renders the Crook et al system too slow for use in high-speed packaging lines for two reasons. First, the stop which at least momentarily terminates the advancement of the items being conveyed renders the conveyence slow. Perhaps more importantly, the actual stopping of the items being conveyed requires that the accumulator conveyor operate at relatively slow speeds so as to avoid potentially harmful banging between the items as they accumulate behind the stop. The slower the accumulator conveyor runs, the more difficult it is to maintain a backlog at the stop and this in turn requires that the packaging machine in a highspeed packaging line operate at a slower rate so that the demand of the packaging machine does not exceed the backlog on the accumulator conveyor.
The Crook et al system is also poorly suited for high-speed packaging lines since it relies upon the sensing of the dielectric presented by the items being conveyed along the accumulator conveyor through the use of RF energy. Such a system cannot of course accommodate baked goods in foil plates since the foil plates will short out the RF energy. In addition, the servo motor control which is responsive to the dielectric constant represented by the items on the accumulator conveyor is somewhat slow due to the inertia of the servo motor so as to cause time-lags between backlog changes and speed signal changes to the speed controller of the soap extruder. This means that the Crook et al system is slow to react to changes in the backlog which could be critical in high-speed packaging lines. Also, the Crook et al system of control is prone to overshoot and oscillation in response to any change in the backlog on the accumulator belt. It is therefore somewhat unstable.